An Andover woman accused of embezzling more than $500,000 from her elderly friend will remain imprisoned following a decision yesterday by the state Supreme Court in which they upheld the bulk of her 2011 convictions.

In so doing, the court also ruled for the first time that someone who shares a joint bank account can be found guilty of theft.

Karen Gagne, 58, was convicted two years ago on 11 theft charges and was sentenced to 10 to 20 years in prison for siphoning the funds from Alice Jane Fair of Concord between 2007 and 2009. A jury decided Gagne had unlawfully sold Fair’s assets and drained the bank account she shared with the Pleasant View Retirement resident – an account created so Gagne could help pay Fair’s bills.

The court yesterday upheld all but two of those convictions, concluding there was enough evidence to convict Gagne on the nine counts regarding the joint account. The two charges they reversed concerned a personal account established before the joint account was created in 2007.

Neither of the reversals affects Gagne’s current sentence, said Jeffrey Cahill, the senior assistant attorney general who prosecuted the case, because the sentences they voided are concurrent with the others upheld.

Gagne’s attorney, Brooksley Belanger, said her team was reviewing the court’s decision and hadn’t yet determined whether to appeal it at a higher level.

Gagne and Fair had lived together at one point after Fair’s husband died, but their friendship splintered in the late 1990s due to an undisclosed financial dispute, according to court testimony. Gagne re-entered Fair’s life in 2006, after the elderly woman had been in a car accident that led her to move into the Concord retirement home. Gagne began driving her to and from appointments, taking her to lunch and paying her bills after Fair fired her accountant, allegedly at Gagne’s behest.

The two opened the joint account the following year. Prosecutors said Gagne used the funds she stole to pay off debt and purchase a home.

In concurring with that argument yesterday, the high court ruled for the first time that someone who shares a joint account can be convicted of stealing by making unauthorized withdrawals.

“Here, the victim testified that the purpose of the joint account was to allow the defendant to better assist her with ‘big bills,’ ” Justice James Bassett wrote. “The victim further testified that she never authorized the defendant to make withdrawals from the joint account for her own needs. Accordingly, we conclude that the evidence was sufficient that the funds in the joint account were the ‘(p)roperty of another.’ ”

Cahill called the decision significant “for law enforcement and for those that investigate cases of financial exploitation.”

Fair died June 30 at Pleasant View.

(Jeremy Blackman can be reached at 369-3319, jblackman@cmonitor.com or on Twitter @JBlackmanCM.)